The invention relates to measurement systems. In particular, the invention relates to extending the dynamic range of lower power signal measurements in the presence of higher power signals.
A critical facet of the design and manufacture of modem communications and related signal transmission systems is the measurement and characterization of signal distortion introduced by the elements that make up the system. All system elements, most notably active devices, such as amplifiers, have non-ideal operational characteristics. These non-ideal operational characteristics can and do distort the signals that pass through or are processed by the elements of the system. The signal distortion introduced by the non-ideal characteristics of the system elements often interferes with the operation of the system. Measurement, characterization and control of system element-related distortion are of paramount importance in most transmission system design and manufacturing activities.
Modern communications systems, especially state-of-the-art wideband systems, are particularly sensitive to signal distortion and its effect on performance. These systems and their designers are faced with ever-increasing demands for greater bandwidth in a finite spectrum and so, must contend with ever-tightening specifications associated with system element-related signal distortion. The ability to perform accurate measurement and characterization of the stimulus/response distortion effects of devices and elements used in the system is a vital consideration in determining the ultimate performance of the system.
Chief among the non-ideal characteristics exhibited by typical devices used in communications systems are non-linear effects. A non-linear effect is defined as the stimulus/response performance of a device that is not completely described by a linear equation. Generally, non-linear effects give rise to signal distortions in the form of a spurious frequency response. That is to say that the device by its operation introduces spectral components into the signal passing through the device that are unwanted and not consistent with the linear operation of that device. Generally, for devices that are classified as linear or quasi-linear, power levels associated with the spurious responses are much lower or smaller than that of the primary or linear response signal. For example, a third order spur from a two-tone measurement might be xe2x88x9260 dBc for a given signal power level relative to the linear response signal. In other words, the spur level is 1,000,000 times smaller than the desired, linear response signal. However, even though the spurious response of a given device is often very small compared to its linear response, the spurious response can have a profound effect on the performance of the system as a whole.
A number of conventional measurement methodologies are used to measure and characterize the non-linear performance of devices used in a communications system. Most of these measurement methodologies either attempt to directly measure an aspect of the non-linear performance of a device or attempt to infer the non-linear performance through an indirect means. Generally, the indirect methodologies focus on measuring the effect of the device non-linearities on some aspect of system performance and therefore, are often referred to as xe2x80x9csystem levelxe2x80x9d measurements. Among the direct measurement methodologies are the 1 dB compression point test, the two tone and multi-tone intermodulation response tests, and saturated power tests. Indirect or system level measurements include such things as the bit error rate measurement, eye patterns or eye diagrams, and the adjacent channel power ratio (ACPR). The ACPR is particularly important for modern, wideband code division multiple access (W-CDMA) systems.
The 1 dB compression point test measures the point at which an input or stimulus power level produces an output power level response that deviates from a linear response by 1 dB. The two-tone and multi-tone tests measure the relative level of a particular spurious response or set of responses compared to the level of the linear or fundamental response. These tests are used to predict or infer the so-called second order, third order, and n-th order intercept points in amplifiers. The saturated power test measures the performance of the device at very high input power levels. As pointed out above, all of these direct measurement methodologies attempt to focus on a particular non-linear characteristic (e.g. second order intercept point). Generally, the measured non-linear characteristics are used to infer the effect that the non-ideal performance of a device will have on a signal passing through the system incorporating the device.
In contrast, the indirect measurements focus on a system level performance parameter. In the indirect measurement methodologies, the sum-total of all of the non-linear performance characteristics of a device are tested or measured simultaneously in as much as they affect the performance parameter being measured. For example, a bit error rate test characterizes how a device or series of devices impacts the rate of bit errors at various stimulus signal-to-noise ratios (SNR) for a digital transmission system. The ACPR measures the amount of power that xe2x80x9cleaksxe2x80x9d from one channel of a system to an adjacent channel as a result of the non-ideal performance of a device under test (DUT). No attempt is generally made to identify what non-ideal performance effect of the DUT is causing the observed performance in the indirect measurements. On the other hand, the data generated by the indirect measurements are generally more closely related to the actually performance parameters of the system as a whole.
In both the direct and indirect measurement methodologies, the goal is to accurately measure the performance parameter and compare that measured value to a system specification or to predict system performance from the measured performance parameter. The sensitivity, dynamic range, and accuracy of such measurements are always of concern to system designers and system manufacturers.
The difficulty that is encountered with many measurement systems used to perform the direct and indirect measurements of device performance is that the measurement systems used to perform the measurements often exhibit inherent non-linear and/or spurious performance characteristics themselves. The inherent non-ideal performance of the measurement systems can limit the dynamic range and accuracy of the tests being performed.
For example, a pair of signal generators used in a two-tone test may generate spurious harmonic signals in the frequency range of the intermodulation product that is being measured. The presence of these spurious signals can limit the minimum level of a given intermodulation product that can be measured by the measurement system. Pre-amps and detectors used in the measurement system can have non-linear performance characteristics that produce spurious signals that interfere with the intended measurements. At the very least, it may be difficult or impossible to make accurate measurements of the amplitude or power level of small or very small signals in the presence of a large, linear response signal.
These inherent, non-ideal characteristics of the measurement system mean that the sensitivity or minimum level of the measurements taken therewith is instrument-limited. The ideal situation is to have measurements that are DUT-limited instead of instrument-limited since it is the non-ideal characteristics of the DUT that are of interest. The ultimate result of the presence of non-ideal characteristics in the measurement system is an effective limitation in the dynamic range of the measurement system, which thus limits the ability of the system to make accurate measurement of very low-level DUT-related distortion signals.
FIG. 1A illustrates a block diagram of a conventional measurement system that can be used for either direct or indirect measurements. The measurement system comprises a signal source and a measurement processor. The device under test (DUT) is connected between the signal source and the measurement processor. The signal source produces a test signal. The test signal is applied to the DUT. The measurement processor receives and processes the signal after it passes through the DUT. Examples of typical signal sources include voltage controlled oscillators, signal synthesizers, and arbitrary waveform generators. Typical measurement processors include power meters, oscilloscopes, and spectrum analyzers. FIG. 1B illustrates a typical result from a two-tone measurement. While the block diagram illustrated in FIG. 1A depicts a transmission measurement of the DUT, one skilled in the art would readily recognize that with minor modifications, a similar measurement system could be used for reflection measurements as well.
Accordingly, it would be advantageous to have an apparatus and method that compensated for the non-ideal characteristics of the measurement system elements. In particular, it would be advantageous to have an apparatus and method that could facilitate measuring very small spurious signal levels without or with less interference from the linear response signal and spurious signals from the signal source. Such an apparatus and method would extend the dynamic range of existing measurement systems, facilitating low-level distortion measurements and improving the accuracy of the measurement performed therewith. Such a dynamic range extension apparatus and method would solve a long-standing need in the areas of communications and signal transmission system device test, measurement and characterization.
The present invention is a novel dynamic range extension apparatus and method that compensate for the non-ideal characteristics of measurement system elements that can interfere with measurements of a device under test. In particular, the dynamic range extension apparatus and method of the present invention facilitate low-level distortion measurements of the device under test and improves the accuracy of such measurements using a novel cancellation signal approach.
In one aspect of the present invention, a dynamic range extension apparatus is provided. The dynamic range extension apparatus extends the dynamic range of measurements performed on the device under test. The dynamic range extension apparatus of the present invention has an input port for accepting an input test signal from a signal source and an output port for delivering an output signal to a measurement processor. The apparatus further comprises a signal splitter having an input and two outputs. The apparatus still further comprises a cancellation path having an input connected to a first output of the signal splitter and a test path having an input connected to a second output of the signal splitter. The apparatus yet further comprises a signal combiner having a first input connected to an output of the cancellation path, a second input connected to an output of the test path and an output. The signal splitter is located between the input port of the apparatus and the inputs of the test and cancellation paths. The signal combiner is located between the outputs of the test and cancellation paths and the output port of the apparatus. The device under test is inserted in the test path during a measurement cycle.
In the dynamic range extension apparatus of the present invention, a signal from the source is split by the signal splitter into two signals, a first split signal passes through the cancellation path and a second split signal passes through the test path. The first split signal, or cancellation signal, is phase shifted by the cancellation path. The second split signal, or test signal, passes through the device under test producing a response signal that includes a main or linear response portion or signal and a distortion portion or signal. The response signal is attenuated in the test path where the attenuation is approximately equal to a gain in the device under test. At the outputs of the cancellation path and the test path, respectively, the cancellation signal and the attenuated response signal are combined by the signal combiner prior to being delivered to the measurement processor. Advantageously, the cancellation signal cancels or removes some or all of the linear response portion of the response signal while leaving the distortion portion of the response signal. By canceling the linear response signal, the present invention can reduce the dynamic range requirements of the measurement processor. Moreover, some or all of any non-ideal spurious signals present in the input test signal from the signal source are also cancelled by the cancellation signal upon signal combining.
In another aspect of the present invention, a method of extending a dynamic range of a measurement performed on a device under test is provided. The method comprises the step of splitting an input test signal into a first signal and a second signal. The first signal enters a cancellation path where it is phase shifted to produce a cancellation signal. The second signal enters a test path where it is applied to the device under test to produce a response signal containing a main or linear response portion or signal and a distortion response portion or signal. The response signal is then attenuated. The method further comprises the step of combining the cancellation signal and the attenuated response signal to produce an output signal. The step of combining the cancellation signal with the attenuated response signal results in the output signal in which the main or linear response portion is largely cancelled, or at least greatly reduced in level, while advantageously, the distortion response portion is relatively unaffected. Thus, the low-level distortion response signal of the device under test can be more readily measured using the method of the present invention.
In yet another aspect of the present invention, a measurement system having extended dynamic range and improved measurement accuracy for measurements of low-level distortion signals produced by a device under test is provided. The measurement system comprises a signal source for producing an input test signal, an apparatus for extending dynamic range, and a measurement processor for processing an output signal from the apparatus. The dynamic range extension apparatus has an input port and an output port. An output of the signal source is connected at the input port of the apparatus and an input of the measurement processor is connected at the output port of the apparatus. The apparatus comprises a signal splitter having an input and two outputs. The apparatus further comprises a cancellation path having an input connected to the first output of the signal splitter and a test path having an input connected to the second output of the signal splitter. The apparatus still further comprises a signal combiner having a first input connected to an output of the cancellation path, a second input connected to an output of the test path and an output. The apparatus for extending dynamic range receives the input test signal from the signal source and the signal splitter splits the signal into two signals. A first split signal passes through the cancellation path and a second split signal passes through the test path, where the device under test is inserted for measurement. The second split signal is applied to the device under test in the test path. The device under test produces a response signal that is attenuated at the output of the test path. The first split signal is phase shifted to produce a cancellation signal at the output of the cancellation path. The cancellation signal and the attenuated response signal from the test path are combined in the signal combiner to produce an output signal at the output port of the apparatus. The output signal from the apparatus is received by the measurement processor. The measurement processor accepts the output signal and measures the low-level distortion signals within the output signal.
The dynamic range extension apparatus, method and system of the present invention are capable of broadband operation. A prototype apparatus and system have been constructed with a nominal frequency range of DC xe2x88x928 GHz. However, there is no limitation to such a frequency range with the present invention. Moreover, advantageously, the dynamic range extension apparatus, method and system can obviate the need for high performance signal sources and high performance measurement processors (e.g., spectrum analyzers) while still allowing high accuracy, high dynamic range measurements to be performed.